Hierarchical Fe2O3@Co3O4 nanowire array anode for high-performance lithium-ion batteries

Journal of Power Sources 240 (2013) 344e350

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Hierarchical Fe2O3@Co3O4 nanowire array anode for high-performance lithiumion batteries Q.Q. Xiong a, X.H. Xia b, J.P. Tu a, *, J. Chen a, Y.Q. Zhang a, D. Zhou a, C.D. Gu a, X.L. Wang a a

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore

h i g h l i g h t s
We prepared Fe2O3@Co3O4 array by the hydrothermal and sacrificial hydrolysis method.
The porous Co3O4 nanowire act as the core and Fe2O3 nanoparticle as the shell layer.
The electrode shows high capacity, good cycle stability and enhanced rate performance.
This strategy can be generalized to construct other hybrid nanostructures.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 February 2013 Received in revised form 26 March 2013 Accepted 9 April 2013 Available online 19 April 2013

A hierarchically Fe2O3@Co3O4 nanowire array is prepared by the aid of hydrothermal synthesis and sacrificial hydrolysis method. The obtained nanowire array shows hierarchical porosity and large surface area. The resulted Fe2O3@Co3O4 nanowire array is evaluated as an anode material for Li ion batteries, which exhibits high capacity and good cycle stability (1005.1 mAh g1 after 50 cycles at a current density of 200 mA g1) and an excellent rate performance, mainly owing to the unique hierarchical nanowire architecture and an elegant synergistic effect of two electrochemically active materials. The developed strategy can be readily generalized to construct other multifunctional hybrid nanostructures, which will be promising materials for high-performance electrochemical devices. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Nanowire array Ferric oxide Cobalt tetroxide Hierarchical structure

1. Introduction With increasing demand in electric energy storage for electric vehicles and modern electronics, lithium-ion batteries (LIBs) as a fast-developing technology have attracted extensive research interest due to their fascinating advantages of high voltage, high energy density, long cycle life and good environment compatibility [1e13]. The pursuit of high-performance LIB never stops and there are continuous demands to develop LIBs with higher power and energy densities. However, current commercialized graphite anode has a relatively low theoretical capacity of 372 mAh g1. In the past decades, great efforts have been devoted to seeking for alternative anode materials for LIBs to improve their energy densities. Nowadays, transition metal oxides with variable valence have been

* Corresponding author. Tel.: þ86 571 87952856; fax: þ86 571 87952573. E-mail addresses: [email protected], [email protected] (J.P. Tu). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.04.042

widely investigated as candidates in view of their multiple oxidation states to meet the ever-growing performance demands [14e 25]. However, the application of transition metal oxides is still hampered by their intrinsic poor electrical conductivity, aggregation of active particles during cycling and large volume variation [26e28]. And there has been limited success in producing welldefined nanostructures with excellent Li storage capabilities and high power density. One promising route is to scrupulously design nanostructured electrodes and smart hybridization of bespoke materials, namely, to combine different active metal oxides into the single electrode to take the advantages of both components and offer special properties through the reinforcement or modification of each other. Recently, great attention has been paid to hybridize nanostructured transition metal oxides with other active/inactive materials to enhance the electrochemical performance [29e32]. In particular, one-dimensional (1D) nanowire heterostructures arouse considerable research interest due to their rich accessible

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75 mL of distilled water. Then the homogeneous solution was transferred into Teflon-lined stainless steel autoclave liners. A piece of clean nickel foam with 3  7 cm2 in size was immersed into the reaction solution. Its top side of the substrate was uniformly coated with a polytetrafluorethylene tape to prevent the solution contamination. The linear was sealed in a stainless steel autoclave and maintained at 110  C for 7 h and then annealed at 500  C for 2 h in Ar. Then, the nanowire array was synthesized by a simple chemical bath deposition method. The solution was prepared by dissolving 0.6 g Zn(NO3)2$6H2O, 0.15 g NH4F, and 0.6 g CO(NH2)2 in 75 mL of distilled water. Previously prepared film was immersed into the reaction solution at 50  C for 15 h. Afterward, the sample was annealed at 500  C in Ar for 2 h. For the fabrication of Fe2O3@Co3O4 nanowire array, the assynthesized ZnO@Co3O4 nanowire array was first placed into a 50 mL solution containing 0.27 g of Fe(NO3)3$9H2O and kept still at room temperature for 10 h. After the immersion, it was taken out, dried in air, and treated at 450  C in Ar for 5 h. The mass ratio of Co3O4 and Fe2O3 is around 3: 4. 2.2. Characterization

electroactive sites, fast Liþ diffusion and potential synergetic properties or multifunctionalities [30,3335]. Co3O4 and Fe2O3, two important members of the metal oxide family, have been the two most widely studied anode materials due to their high theoretical capacity, nontoxicity, and abundance [3638]. To date, different nanostructures of Co3O4 and Fe2O3 such as spheres, micro-flowers, nanowires and particles have been synthesized and applied for LIBs. Nevertheless, all these works focus on single Co3O4 or Fe2O3 electrodes, there are few reports on electrochemical performance of their 1D integrated electrodes by rationally combining merits of Co3O4 and Fe2O3. In this present work, we present a simple and cost-effective method to synthesize hierarchical Fe2O3@Co3O4 nanowire array based on the sacrificial template-accelerated hydrolysis mechanism using ZnO as the intermediate template [39], where the porous Co3O4 nanowires act as the “core” and ultrathin Fe2O3 nanoparticles as the “shell” layer. Impressively, the electrochemical performance of hierarchical Fe2O3@Co3O4 nanowire array is greatly improved with high capacity and excellent cycling life. This enhancement is mainly benefited from the multiple apparent advantages of the nanowire array configuration, such as high surface areas, short ion diffusion path, and direct growth on conductive substrate. 2. Experimental

The morphology and microstructure of products were characterized by X-ray diffractometer (XRD, Rigaku D/max 2550 PC, Cu Ka), scanning electron microscopy (SEM, Hitachi S-4700) and transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). 2.3. Electrochemical investigation The electrochemical tests were performed using a coin-type half cell (CR 2025). Test cells were assembled in an argon-filled glove box with the metallic lithium foil as the counter electrode, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DME) (1: 1 in volume) as the electrolyte, and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator. The galvanostatic chargedischarge tests were conducted on a LAND battery programcontrol test system at certain current densities between 0.01 and 3.0 V at room temperature (25  1  C). Cyclic voltammetry (CV) was performed on a CHI660C electrochemical workstation in the potential from 0 to 3.0 V (vs. Liþ/Li) at a scanning rate of 0.1 mV s1. In the electrochemical impedance spectroscopy (EIS) measurement, the excitation voltage applied to the cells was 5 mV and the frequency range was from 100 kHz to 10 mHz. In this experiment, in the term of capacity of the Fe2O3@Co3O4 nanowire array, it is based on the quality of both the active materials.

2.1. Preparation of hierarchical Fe2O3@Co3O4 nanowire array 3. Results and discussion First, self-supported Co3O4 nanowire array was prepared by a typical hydrothermal synthesis method. The solution was prepared by dissolving 0.6 g Co(NO3)2$6H2O, 0.15 g NH4F, and 0.6 g CO(NH2)2 in

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The crystal structure of the Fe2O3@Co3O4 nanowire arrays is analyzed by XRD, with the result shown in Fig. 1. The composite

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Fig. 2. A schematic illustration of the formation of hierarchical Fe2O3@Co3O4 nanowire array.

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Fig. 3. Top and cross-sectional SEM images of (a, b) Co3O4 nanowire array and (c, d) hierarchical Fe2O3@Co3O4 nanowire array.

nanowire array shows a mixed XRD pattern of cubic Co3O4 (JCPDF card no. 42-1467) and hexagonal Fe2O3 (JCPDF card no. 33-0664). No peaks of other impurity can be observed, suggesting that Fe(OH)3 is completely transformed to Fe2O3 after the annealing process. The diffraction peaks at 31.3, 36.9, 38.5, 59.4 and 65.2 are

associated with the 220, 311, 222, 511 and 440 reflections of Co3O4, and the diffraction peaks at 24.1, 33.1, 35.6, 49.5, 54.1, 62.4 and 64.0 are correlated to the 012, 104, 110, 024, 116, 214 and 300 reflections of Fe2O3, indicating successful synthesis of well-oriented Fe2O3@Co3O4 nanowire array.

Fig. 4. TEM images of (a) Co3O4 nanowire and (b) HRTEM image of Co3O4 nanowire and (c) hierarchical Fe2O3@Co3O4 nanowire scratched from Ni foam and (d) HRTEM image of hierarchical Fe2O3@Co3O4 nanowire.

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Fig. 2 illustrates schematically the formation process of Fe2O3@Co3O4 nanowire array on conductive substrate. The phase purity of samples prepared at different step is illustrated by XRD analysis (Fig. 1). First, Co3O4 nanowire arrays are grown on Ni foam

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using a typical hydrothermal synthesis method, according to previous works [28,36]. These Co3O4 nanowire arrays acting as the backbone, are then subjected to impregnation with Zn2þ aqueous solution and subsequent annealing in Ar, which leads to the uniform coating of ZnO layer on the nanowire surface. Herein the ZnO layer is purposely designed as an interfacial reactive template to grow Fe2O3 nanostructures. Then, the previously prepared arrays are immersed in an aqueous solution containing Fe3þ at room temperature. Simultaneously, the ZnO layer is etched by Hþ, which comes from the hydrolysis of Fe3þ. As a result, progressive hydrolysis and template dissolution eventually result in the precipitation of Fe(OH)3 emerging on the scaffold of Co3O4 nanowire arrays [39]. After this step, the resulting Fe(OH)3 is calcinated in Ar and transformed into ordered Fe2O3@Co3O4 nanowire array. This method provides a new principle for designing integrated nanowire arrays without any porous membranes (AAO or polymers). The morphologies of the Co3O4 nanowire array and Fe2O3@Co3O4 nanowire array are characterized using SEM. Fig. 3a shows the skeletons of the nickel foam are uniformly covered by the slim Co3O4 nanowires with diameters of 80e100 nm and length up to around 8 mm, which are homogeneously aligned and separated apart. The top of the Co3O4 nanowires is sharp (Fig. 3b). The length of nanowires could be easily controlled by the growth time. With two components integrated, the morphology of the sample does not change much, maintaining the nanowire array structure, but the surface of the composite nanowires becomes a little coarser

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Fig. 6. CV curves of (a) Co3O4 nanowire array and (b) hierarchical Fe2O3@Co3O4 nanowire array for the first and second cycles at a scan rate of 0.1 mV s1 in the potential range of 0e 3.0 V (versus Li/Liþ), and the chargeedischarge profiles of (c) Co3O4 nanowire array and (d) hierarchical Fe2O3@Co3O4 nanowire array between 0.01 and 3.0 V at a current density of 100 mA g1.

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(Fig. 3c and d). The Co3O4 nanowire surface is uniformly decorated by many ultrathin Fe2O3 nanoparticles. Thus, nearly all the composite nanowires are highly accessible to electrolytes for energy storage due to the presence of convenient diffusion channels. The detailed morphological and structural features of the Co3O4 nanowire and Fe2O3@Co3O4 nanowire array are also examined by TEM. The Co3O4 core nanowire is highly porous, composed of nanocrystallites of 10e20 nm and pores of 4e6 nm (Fig. 4a). The HRTEM examination of a Co3O4 nanowire shown in Fig. 4b reveals a distinct set of visible lattice fringes with inter-planar spacing of 0.29 nm and 0.25 nm, corresponding to the (220) and (311) plane of cubic Co3O4, respectively. Fig. 4c shows an individual hybrid nanostructure in which ultrathin Fe2O3 nanoparticles uniformly cover the surface of the porous Co3O4 nanowire. A close examination of the exposed profile reveals that the outer Fe2O3 shell is 80e 90 nm in thickness. Moreover, the thickness of the Fe2O3 shell can be easily varied by controlling the reaction time of Zn2þ. The HRTEM image of outer Fe2O3 shell shown in Fig. 4d reveals the inter-planar spacing of 0.37 and 0.25 nm, corresponding to the (012) and (110) plane of hexagonal Fe2O3. Another phenomenon is that, even after a long time of sonication during the preparation of the TEM specimen, the Fe2O3 shell is still strongly attached to the Co3O4 nanowire, suggesting the strong interaction of the hybrid nanostructure. The nitrogen adsorption and desorption measurements were performed to estimate the BrunauereEmmetteTeller (BET) surface area and pore size distribution of the hierarchical Fe2O3@Co3O4 nanowire. As shown in Fig. 5, the isotherm of the hierarchical Fe2O3@Co3O4 nanowire exhibits a hysteresis loop at the P/P0 ranges of 0.6e1.0, indicating the presence of mesopores. The BET surface area of the hierarchical Fe2O3@Co3O4 nanowire is calculated to be 21.31 m2 g1. The plot of pore size distribution determined by the BarretteJoynereHalenda (BJH) method (Fig. 5, inset) shows the main pore size of around 6.9 nm and 27.6 nm. The mesopores on the hierarchical nanowire can be attributed to the interspaces of the constituent particles. The result is in good agreement with the value determined by HRTEM observations above. The relatively large specific surface area and porosity offer large materials/electrolyte contact area and promote Liþ diffusion. We investigated its ability to reversibly react with lithium in comparison with that of Co3O4 nanowire to demonstrate the potential use of hierarchical Fe2O3@Co3O4 nanowire array as an anode for LIBs. Fig. 6a and b show the CV curves of the Co3O4 nanowire and Fe2O3@Co3O4 nanowire arrays for the first two cycles in the potential range from 3.0 to 0.0 V (vs. Li/Liþ) at ambient temperature at a scan rate of 0.1 mV s1, respectively. Apparently, a pair of redox peaks is observed for the Co3O4 nanowire array electrode, corresponding to the extraction and insertion of lithium ions. The reduction peak corresponds to a multistep electrochemical reaction of Co3þ to Co2þ and metal Co0. In the anodic scan, one broad peak in the range of 2e2.5 V is recorded, which is ascribed to the reversible oxidation of metal Co to cobalt oxide [28,40]. For the Fe2O3@Co3O4 nanowire array, it is remarkable that in addition to the peaks from Co3O4, indicating the efficient utilization of the underlying Co3O4 nanowire despite covered by the Fe2O3 nanoparticles, there is another pair of redox peaks (Fig. 6b). The reduction peak at about 0.5 V is observed in the first cathodic scan, which can be attributed to the reduction of Fe3þ to Fe0 and the irreversible reaction with the electrolyte. Meanwhile, the anodic peak at about 1.6 V corresponds to the reversible oxidation of Fe0 to Fe3þ, which agrees well with early studies [4144]. It is interesting that the area integrated within the currentepotential curves greatly increases for the Fe2O3@Co3O4 nanowire array electrode, leading to a much larger capacity. Fig. 6c and d show the first two galvanostatic chargee discharge curves for the two electrodes between 0.01 and 3.0 V at a current density of 100 mA g1. The Fe2O3@Co3O4 nanowire array

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reaches a high initial discharge capacity of 1586.9 mAh g1, substantially better than that of Co3O4 nanowire array (1226.7 mAh g1). These values are much higher than the theoretical capacity, which can be attributed to be the formation of solid electrolyte interphase (SEI) films on the nanowire arrays during the discharge process [45]. The discharge capacity of two electrodes drops at the second cycle, corresponding to an irreversible capacity loss due to the formation of SEI films, the decomposition of electrolyte and the formation of the irreversible Li2O [43]. However, the Fe2O3@Co3O4 nanowire array electrode shows smaller irreversible capacity, indicating that the hierarchical nanowire array leads to better reversibility in the chargeedischarge processes. EIS test was also carried out to prove the good performance of hierarchical nanowire array. Fig. 7a shows the Nyquist plots of both the electrodes at room temperature in the frequency range of 100 KHz to 10 mHz after 3 cycles in the fully charged state. They have the same shapes of Nyquist plots, composed of a depressed semicircle where a high-frequency semicircle and a mediumfrequency semicircle overlap each other and a long low-frequency line. The intercept on the Z real axis in the high-frequency region corresponds to the resistance of electrolyte (Rs). The semicircle in the middle frequency range indicates the charge-transfer resistance (Rct), relating to charge transfer through the electrode/electrolyte interface. The inclined line in the low-frequency region represents the Warburg impedance (Zw), which is related to solid-state diffusion of Liþ in the electrode materials [46]. As one can observe that

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after 3 cycles, there are no obvious changes of semicircle between the two electrodes. Fig. 7b presents the Nyquist plots of the electrodes after 50 cycles. Obviously, the Fe2O3@Co3O4 nanowire array electrode has smaller semicircle than the Co3O4 nanowire array. Therefore, it is suggested that the interparticle resistance of the electrode is suppressed by the hierarchical nanostructure, and consequently, the value of Rct for the Fe2O3@Co3O4 nanowire array electrode is smaller than that of the Co3O4 nanowire, indicating that the composite nanowire arrays are beneficial to enhance the reaction kinetics and better cycling performance of the cells during the charge/discharge process. Fig. 8 shows the cyclability of the Co3O4 nanowire and Fe2O3@Co3O4 nanowire array electrodes at a current density of 200 mA g1. The specific reversible capacity of Fe2O3@Co3O4 nanowire arrays after 50 cycles is 1005.1 mAh g1, which is much higher than that of Co3O4 nanowire (483.9 mAh g1). The reversible capacity of the Fe2O3@Co3O4 nanowire array remained almost constant since the third cycle, with coulombic efficiency above 98% from the 5th cycle. After 50 cycles, the Fe2O3@Co3O4 nanowire array could sustain 75.6% capacity of the 2nd cycle, compared with 57.7% for Co3O4 nanowire array. The substantial enhancement of Liþ storage capacity and stability of hybrid nanowire array is attributed to its composite nanostructure, which provides a short ion diffusion path, maintains the structural integrity of the core by wrapped Fe2O3 nanoparticles. In addition, both the Fe2O3 and Co3O4 are active materials to reversibly undergo redox reactions with Li, hence contributing to high Liþ storage capacity. In addition to the improved cycling performance, the Fe2O3@Co3O4 nanowire array also show significantly enhanced rate performance compared with the Co3O4 nanowire (Fig. 9). The specific capacities of Fe2O3@Co3O4 nanowire array at different chargee discharge current densities are always higher than those of Co3O4 nanowire. With increasing the current density from 200 mA g1 to 5000 mA g1, the discharge capacities of the two electrodes decrease gradually, indicating the diffusion-controlled kinetics process for the electrode reaction. At a current density as high as 5000 mA g1, the Fe2O3@Co3O4 nanowire array electrode is capable of delivering a stable capacity of about 760.0 mAh g1 after 20 cycles of variable charging rate. Upon decreasing the current density to 200 mA g1, nearly 84.5% of the capacity of the second cycle at 200 mA g1 (about 1050.0 mAh g1) can be recovered. Fig. 10 shows the SEM images of the Co3O4 and hierarchical Fe2O3@Co3O4 nanowire arrays after 10 cycles at a current density of 200 mA g1. The surface of Fe2O3@Co3O4 nanowire array is much rougher (Fig. 10b). Fortunately, the ultrathin Fe2O3 nanoparticles attaching to the Co3O4 nanowire almost exhibit no agglomeration. The morphology of Fe2O3@Co3O4 nanowire array remains well after the lithiation/delithiation cycles, compares to that of Co3O4

Fig. 10. SEM images of Co3O4 nanowire array and hierarchical Fe2O3@Co3O4 nanowire array after 10 cycles at a current density of 200 mA g1.

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(Fig. 10a), indicating that hierarchical nanowire array structure is beneficial to relax the volume expansion and alleviates the structure damage during cycling. The composite array directly grown on conductive Ni foam and the structural stability during cycling may be responsible for the enhanced rate performance, which implying that it is a promising anode material for high energy or high power density LIBs. 4. Conclusions In summary, we have successfully fabricated hierarchical Fe2O3@Co3O4 nanowire array based on the sacrificial templateaccelerated hydrolysis mechanism using ZnO as the intermediate template. The Fe2O3@Co3O4 nanowire array exhibits good cycling performance (1005.1 mAh g1 after 50 cycles at 200 mA g1) and improved rate performance (788.9 mAh g1 at 5000 mA g1). Such intriguing capacity behavior is attributed to the hierarchical composite nanowire array configuration and the synergistic effects of the combined electrochemical contributions from the Co3O4 nanowire core and the ultrathin Fe2O3 nanoparticle shell. The utilization of advanced hierarchical integrated array structure is suggested as a promising strategy toward the development of transition metal oxides as high-performance electrode materials for LIBs. Acknowledgments This work is supported by the National Science and Technology Support Program (2012BAK30B04) and Key Science and Technology Innovation Team of Zhejiang Province (2010R50013). The assistances of Dr. Xu-yang Zhao and Shao-jun Shi for SEM analysis and Mr. Xin-ting Cong for TEM analysis are grateful acknowledged. References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [2] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nat. Mater. 4 (2005) 366. [3] M. Armand, J.M. Tarascon, Nature 451 (2008) 652. [4] Y.J. Mai, S.J. Shi, D. Zhang, Y. Lu, C.D. Gu, J.P. Tu, J. Power Sources 204 (2012) 155. [5] A. Brandt, A. Balducci, J. Power Sources 230 (2013) 44e49. [6] J.Y. Xiang, J.P. Tu, L. Zhang, Y. Zhou, X.L. Wang, S.J. Shi, Electrochim. Acta 55 (2010) 1820. [7] V. Aravindan, P.S. Kumar, J. Sundaramurthy, W.C. Ling, S. Ramakrishna, S. Madhavi, J. Power Sources 227 (2013) 284. [8] Y. Lu, J.-P. Tu, Q.-Q. Xiong, J.-Y. Xiang, Y.-J. Mai, J. Zhang, Y.-Q. Qiao, X.-L. Wang, C.-D. Gu, S.X. Mao, Adv. Funct. Mater. 22 (2012) 3927. [9] J.J. Zhang, Y.F. Sun, Y. Yao, T. Huang, A.S. Yu, J. Power Sources 222 (2013) 59. [10] M. Sathish, T. Tomai, I. Honma, J. Power Sources 217 (2012) 85. [11] Y. Lu, J.-P. Tu, Q.-Q. Xiong, H. Zhang, C.-D. Gu, X.-L. Wang, S.X. Mao, Cryst Eng. Comm. 14 (2012) 8633. [12] D. Zhang, X.L. Wang, Y.J. Mai, X.H. Xia, C.D. Gu, J.P. Tu, J. Appl. Electrochem 42 (2012) 263.

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